Encapsulated precision wire-wound resistors



United States Patent 3,051,923 ENCAPSULATED PRECISION WIRE-WOUND RESISTORS Loebe Julie, New York, N.Y., assignor to Julie Research Laboratories, Inc., New York, N.Y. Filed Sept. 22, 1959, Ser. No. 841,516 2 Claims. (Cl. 338-261) The present invention relates to encapsulated precision wire-wound resistors and methods of making these precision resistors with flexible wire terminal leads. More particularly, this invention relates to encapsulated precision wire-wound resistors having flexible wire terminal leads which are readily fabricated and provide high strength against the torsion and tension stresses such as may be imposed upon the leads during mounting of the resistors in circuit assemblies and during usage in mobile equipment.

The precision resistors and methods of fabrication described herein as illustrative embodiments of the present invention provide for firm anchoring of each flexible wire terminal lead onto the bobbin on which the resistance wire is wound. Each flexible terminal lead is secured to the body of the resistor and is also secured to a terminal transition element having an effective conductivity intermediate that of the flexible lead and that of the resistance wire itself. Then, the end of the resistance wire is butt-welded onto this transition element. This construction enables the location of the effective point of connection at each end of the wire with respect to the length of the resistance wire to be determined with an absolute precision better than the diameter of the wire itself. In prior resistors the ends of the resistance wires are connected by arrangements which leave the location of the effective points of termination indefinite by amounts of thirty diameters of wire or more, for example, by beads of solder or the like. My tests have indicated that the effective point of connection between a high resistance wire and a relatively low resistance solder bead tends to travel along the length of the wire with the passage of time and with passage of current, thus changing the effective length of the resistance wire which is in circuit between its terminals. The effective length of the resistance wire in circuit between the transition terminal elements advantageously remains constant during installation and usage in the illustrative embodiments of the invention described herein. This terminal transition element and the flexible wire terminal lead are arranged to isolate the resistance wire itself from any stress and strain effects which may be imposed on the flexible lead.

In the precision resistors described herein as illustrative embodiments of the present invention, the location of the terminations at the ends of the resistance wire is determined with an accuracy commensurate with the diameter of the wire and the precision of the termination is maintained over indefinitely long periods of time both during use and during idleness. In typical resistors embodying the present invention the locations of the terminations are determined with a precision of better than one part per million of the total length of the resistance wire and the resistance wire is isolated from stresses or strains so that its resistance value does not change during mounting or in use.

In accordance with certain prior art practices in making encapsulated precision wire-wound resistors it is necessary to utilize an expensive and time-consuming procedure to obtain the desired final resistance value. The first step of this prior practice is to try to make a main wire-wound resistor having a value as close to but below the desired value as possible; the resistance wire is subjected to an aging procedure to stabilize its resistance 7 characteristics; then the actual value of this main re- 3,051,923 Patented Aug. 28., 1962 sistor is measured after assembly and aging; next an auxiliary trimming resistor wire is added in series with the main resistor; and finally the total value of the composite resistor is measured, often following a second aging for the composite resistor.

The fabrication procedures described herein as illustrative embodiments of the present invention enable a single resistance wire to be utilized for completion of each resistor. This wire is wound on the support, joined by butt-welding at one end to a transition terminal element which has been secured to a flexible terminal lead. Then, this resistance wire is aged by a suitable aging procedure as described below, and finally it is cut to the desired length and its free end is butt-welded to another transitional terminal element which has been secured to the other flexible lead. Advantageously, this procedure provides a resistor whose actual resistance value after assembly is precisely determined in advance of final assembly and which is extremely stable in operation over an indefinitely long time.

Among the further advantages of the methods and apparatus of the present invention are their suitability for making individual highly precise wire-wound resistors having resistance values of any desired amount within the entire range from ten ohms up to ten million ohms and which maintain their effective points of termination precisely positioned so that the resistors are stable in their resistance values over indefinitely long periods of time.

In addition, these resistors and the flexible terminal means described exhibit high torsion and tension strength and ability to withstand vibration. The methods and apparatus of the present invention enable the elimination of the complex arrangements and systems required by the prior art to secure precise results.

In this specification and in the accompanying drawings are described and shown encapsulated precision wirewound resistors and fabrication methods embodying my invention and various modifications thereof are indicated, but it is to be understood that these are given for purposes of illustration in order that others skilled in the art may fully understand the invention and the manner of applying the methods and apparatus in practical use so that they may modify and adapt them in various forms, each as may be best suited to the conditions of a particular resistor environment.

The various objects, aspects, and advantages of the present invention will be more fully understood from a consideration of the following specification in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a precision wirewound resistor having axially extending flexible wire terminal leads. This view shows the assembled resistor prior to the final encapsulation of the resistor;

FIGURE 2 is an axial sectional view of the resistor of FIGURE 1, and shown on enlarged scale;

FIGURE 3 is an axial sectional view of the resistor of FIGURES l and 2 illustrating the final encapsulation procedure;

FIGURE 4 is an axial sectional view generally similar to FIGURE 3 but illustrating an alternative fluid medium surrounding the resistance wire and the method of its application;

FIGURE 5 is an axial elevational view of another precision wire-wound resistor embodying the present invention and illustrating an intermediate step in the fabrication;

FIGURE 6 is an axial sectional view of the resistor of FIGURE 5, being shown in the final stages of encapsution and on larger scale than FIGURE 5 FIGURE 6A is a partial axial sectional view of an embodiment of a resistor similar to the one shown in The resistor 20 shown in FIGURE 1 includes a spool or bobbin 2]. of electrically insulating material having a plurality of flanges 22 integrally formed on a hollow cylindrical core support 24, as seen in FIGURE 2. The resistance wire 26 is wound on the cylindrical support 24 with approximately equal portions of the wire lying in each of the annular winding channels 28 between the respective flanges 22.

In order to reduce the effective inductance of the resistance winding, the directions of winding of the portions of the wire lying in adjacent winding channels is reversed, as will be noted from FIGURE 1. In passing from one winding channel to the next, the wire passes through a slot 30 in the intervening flange and reverses its direction in the next successive channel. These slots 30 are axially aligned and each one extends radially inwardly toward the core 24 for a distance of approximately onethird of the depth of the flange.

Terminal means 31 are provided at opposite ends of the resistor 20 comprising a pair of flexible wire terminal leads 32 secured to respective transition elements 34. These terminal means 31 are firmly anchored at opposite ends of the bobbin 21. Each of the terminal leads 32 is adapted for making external electrical connections and for mounting the resistor in a circuit assembly. The lead is formed of a highly conductive and readily bent wire material, for example, such as soft copper or soft brass, which has a suitable coating of tin or the like for ease of soldering to external connections.

In order to provide a firm anchor for the flexible leads 32 and in order to provide a precisely predetermined position for the effective termination of the resistance wire .26, the termination transition element 34 in the form of a short stifif length of wire is rigidly secured crosswise to each lead 32 at a significant distance from its inner end. It will be noted from FIGURE 2 that the distance from the inner end of the terminal lead 32 to the transition cross piece 34 is suflicient to allow a substantial length of the lead to extend into the axial bore 36 of the core 24. This distance from the inner end of the lead 32 to the transition element 34 is in the range from 7 of an inch to /8 of an inch and is usually commensurate with the axial length of one of the main winding channels 28, thus maintaining a relatively large spacing between the inner ends of the two leads so as to provide a high leakage resistance therebetween.

Each transition terminal element 34 is rigidly secured to its lead 32 by a connection having greater mechanical strength and heat resistance than would be provided by soldering. In this example, each cross element 34 is spot welded onto its flexible lead 32 before the terminal means are assembled with the empty bobbin 21. Instead of spot welding, brazing may be used, but spot welding is found to be quite satisfactory.

In the form of bobbin as used, the outer end surfaces 38 of the two end flanges are flush with the end of the core 24. When the terminal means are assembled with the empty bobbin, the inner end of the lead has quicksetting cement 40 applied thereto and is inserted into the snug fitting bore 36 until the transition element 34 rests against the end surface 38. The cement 40 also secures the cross element 34 to the surface 38, but the subsequent butt-welding of the resistor wire thereto. The

- length of the cross element 34 is preferably just slightly less than the diameter of the bobbin.

In this example,

welded to the transition element 34.

4 the diameter of the bobbin is of an inch and the element 34 is of an inch long.

The element 34 has an effective conductivity intermediate the high conductivity of the flexible lead 32 and the low conductivity of the resistance wire itself. In this way, the element 34 provides a transition between the high resistance of the end of the resistance wire and the very low resistance of the lead 32, thus precisely establishing the effective length of the resistance wire, as will be explained.

In order to provide a precisely predetermined length for the resistance Wire 26, each end of the wire itself is electrically butt-welded to the terminal by a process as described in my copending application Serial No. 649,439, filed March 29, 1957. It has been found to beundesirable for several reasons to attempt electrically to butt-weld the end of the resistance wire 26 directly to the surface of the flexible lead 32. There is markedly greater heat absorption capacity in the largerdiameter lead 32 as compared with the fine wire 26. Also, their thermal conductivities and specific resistivities are widely diflering, and their fusing temperatures differ. Thus, a precise predetermining of the effective point of termination is not obtained by attempting to butt-weld the resistor wire to the terminal lead, for indeterminate and unpredictable amounts of the wire 26 melt away and are consumed before the weld would be completed. This has been found to cause serious inaccuracy in the over-all resistor value.

These problems are advantageously overcome by utilizing the terminal transition element 34 which has an eflective conductivity intermediate the lead 32 and the Wire 26. By utilizing an element 34 of a resistance material having a specific resistivity similar to that of the fine resistance wire but of a diameter comparable to the lead 32, then the element 34 is found to be highly satisfactory for providing the desired transition. In this example, the diameters of the transition element 34 and of the lead 32 are each approximately 40 mils, and the element 34 is formed of resistance material, for example, Manganin resistance Wire, such as is obtained commercially from Wilbur B. Driver Company of Newark, New Jersey.

Generally, the procedure for assembly of the resistor 20 is to butt-weld the end of the resistance wire 26 to the exposed surface of one of the transition elements 34 at a point 42 which is spaced about A of an inch from the lead 32 and on the opposite side of the element 34 from its region of attachment to the lead 32. Then, the resistance wire is wound onto the bobbin. The resistance wire and bobbin are aged, and the free end of the wire is trimmed to the desired length and butt-welded at 42 to the opposite element 34. For example, in making a butt-weld at the second end of the wire when the winding operation has been completed, the wire must be trimmed so as to give the precisely desired length after it has been In order to obtain this precisely desired length after welding, a small allowance of an added increment of length must be made for wire which becomes fused and thus consumed in the butt-welding operation. This allowance is less than .005 of an inch.

After trimming, the enamel or other insulation material is scraped from or otherwise removed from the resistance wire end for a length of about one-quarter of an inch. The bared wire is grasped between a pair of electrically conductive tweezers connected in circuit in series with a current. limiting resistor and a charged capacitor which is connected to the terminal means 31 to which the buttweld is to be made. The tweezers are used to touch the end of the wire 26 substantially perpendicularly against the clear surface of the element 34. An arc is created at the end of the wire as the capacitor is discharged. The magnitude of the current flow through the arc is limited by the series current-limiting resistor to obtain the desired welding action. The current-limiting resistor has a value in the range from 25 to 50 ohms and the voltage on the capacitor prior to discharge is 300 volts. The capacitor has a capacitance value of 1, 2, or 8 microfarads for butt-welding resistance wires of l, 2, or 4 mils diameter, respectively, as explained in said copending application.

As a result of this butt-welding process, a precisely controlled weld is obtained and the desired value of the resistance wire between the two transition elements 34 is advantageously provided with an accuracy of better than one part per million. There is no requirement for the subsequent application of an auxiliary length of resistance wire to reach the desired value.

Suitable resistance wire material is, for example, Evenohm enamel-coated resistance wire, which is obtainable commercially from Wilbur B. Driver Co. of Newark, New Jersey and Karma enamel-coated resistance wire, from Driver-Harris Co. of Harrison, New Jersey. Suitable resistance wires, such as these, have a composition approximately of 75% nickel, chromium, 2.5% aluminum and 2.5% copper. Other suitable resistance wires obtainable from Wilbur B. Driver Co. are: Tophet A having a composition approximately of 80% nickel and 20% chromium; and Tophet C having approximately a composition of 60% nickel, 15% chromium and iron. Another very suitable resistance wire is sold by Driver-Harris Co. under the name Nichrome and has a composition of approximately 60% nickel, 15% chromium and 25% iron.

By following this procedure the operator is enabled to determine in advance of butt-welding the effective point along the length of the resistance wire 26 at which connection is made to the terminal means, with a precision tolerance commensurate with the diameter of the wire itself, usually resulting in an over-all precision of better than one part per million in resistance value. Because the location of the effective points of terminal connections are thirty times more precisely deter-mined, this invention enables the making of resistors containing only one-thirtieth the length of wire as is required by prior art procedures and yet having a precision equal to or better than prior resistors. By virtue of the shorter wires used, these resistors have far less inductance and capacitance than prior resistors of the same precision.

For resistors having a resistance value below 10,000 ohms, it is usually preferable to utilize a resistance wire having a diameter of at least .004 of an inch. In the range between 10,000 ohms and 100,000 ohms, a diameter of .002 of an inch is preferable; and above 100,000 ohms, .001 of an inch is preferable.

It will be appreciated that the terminal means are firmly anchored against tension, torsion, or bending forces. The transition element 34 provides firm anchorage for the terminal means as a whole with respect to tension or torsion stresses applied to the lead 32. The cementing of the end of the lead into the bore 36 secures the terminal against bending forces on the lead 32. Therefore, the portions of the resistance wire near to the terminal means are effectively isolated from the various kinds of stress applied to the projecting ends of the leads.

The transition element 34 has a specific resistivity markedly greater than that of the terminal lead 32 and preferably has a specific resistivity comparable with that of the resistance wire 26. In this example, it is noted that the specific resistivity of the transition element 34 is substantially equal to that of the resistance wire, being formed of resistance wire material, such as Manganin. Thus, because of its larger diameter, the effective conductivity of the transition element 34 is intermediate the conductivity of the wire and that of the terminal lead 32.

After the second butt welding operation has been completed, a snug-fitting sleeve 44 of insulation material similar to that of the bobbin is slid over the bobbin, as is shown in FIGURE 3 or 4-, and the axis of the bobbin is positioned vertically. This sleeve is longer than the bobbin and is positioned symmetrically thereon so that each end of the sleeve projects about /s of an inch beyond the surface 38, forming a shallow cup 39 at each end about the element 34, as illustrated in FIGURE 4. An encapsulating material 46 is poured in to fill this cup and allowed to harden. The material 46 preferably has the same thermal expansion characteristics as the sleeve 44 and forms an integral bond therewith to form an integral enclosure around the resistance wire. After the one end has hardened, then the resistor is turned end for-end and encapsulating material is poured similarly into the other cup and allowed to harden. This completes the assembly of the resistor. It will be noted that substantially the entire length of the resistance wire itself is free from contact with the encapsulating material and thus is free of any stresses which may be imposed upon the bobbin 21 or the sleeve 44. Instead, the resistance wire is bathed by a fluid environment which is confined within the integral inclosure formed by the encapsulating material. In this embodiment the fluid medium comprises the air within the winding channels 28.

By virtue of the fact that the encapsulating material 46 at each end forms an integral bond with the sleeve, there is provided an integral enclosure containing the fluid medium surrounding the winding. In order to form this integral bond at the juncture 47 between the sleeve 44 and the material 46, it is preferred that the material 46 be substantially identical with the material forming the sleeve. For example, the sleeve 44 is formed of a resin material, such as epoxy resin, obtainable from Nor-rich Plastics Corp., of New York City, New York, and the encapsulating material 46 is epoxy resin as is obtained from Houghton Laboratories of Clean, New York.

In the precision resistor 20A shown in FIGURE 4 the fabrication steps are identical to those explained in conjunction with FIGURES 1, 2, and 3 except that an insulating liquid medium, for example, a transformer oil 48, is injected by a hypodermic needle 50 through the slot 30 at one end before the final layer of encapsulation material 46 is applied to this end. The air escapes from the various winding channels 28 by passing upwardly through the aligned slots 30 as the liquid enters through these slots.

In the precision reistor 20B of FIGURES 5, 6 and 7 parts performing functions corresponding with those in the precision resistors 20 and 20A have corresponding reference numbers. Advantageously, the flexible terminal leads 32 are offset from the axis so that the bore 36 can be made open at both ends for mounting purposes. The terminal transition elements 34 are formed of resistance wire material corresponding with that discussed above. They are anchored at each end of the bobbin by bending into =C-shaped anchor portions 50, as shown in FIG- URE 7, each lying within an end groove 52. The other end portions 54 of each transition element is bent out at 42 into a position perpendicular to the plane of the 'C-shaped anchor portion 50. This extending end portion 54 passes out through a notch 56 in the end flange of the bobbin. At a subsequent stage in the assembly, this extending end 54 is cut off flush with the end surface 38 of the bobbin, as will be described further below. It is noted that the notch 56 is aligned with the winding slots 30, thus placing the right-angle Ibend 42 adjacent to the end of the resistance wire. The end of the resistance wire is butt-welded at the point 42. During the buttwelding operations at each end of the resistance wire, the electrical circuit is completed by making contact with the respective end portions 54. Then, the transition elements are each trimmed oil fiush with the end surfaces 38.

In order to connect the flexible wire terminal leads 32, there are provided a pair of notches 58 in the end flanges opposite the notches 56. The inner ends of the leads 32 75 fit through these notches and are bent at 60 to encircle bobbin. bent out before the encapsulating material is poured.

the C-shaped anchor 50, as seen in FIGURE 6, and are soldered thereto.

For purposes of encapsulating the precision resistor 2013, the sleeve 44 is slid into position around the bobbin. A pair of removable plugs 62 are placed in each end or the bore 36, and then the closures 46B of encapsulating material. are cast in place in the respective ends while the axis of the resistor is held upright. After the material 46B has hardened, the plugs 62 are removed, and the resistor is completed.

1 .FIGURE 8 shows a precision resistor 200 which is identical wtih the resistor 20B except that an insulating liquid .48 is injected through the notch 56 and through the slots 30 before the second end of the resistor is enclosed by the material 46B.

I In the resistors of 20B and 20C the width of the ribs 22 is made equal to the Width of the winding channels 28, and a total of ten winding channels is provided. This construction advantageously reduces the over-all shunt capacitance and hence the time-constant for the resistor. The resistor 20D shown in FIGURE 6A is identical with the resistor 20 B or with the resistor 20C except that the two flexible terminal leads are brought out in a radial direction parallel with one another. To accommodate these terminal leads, the sleeve 44 is formed witha pair of axially aligned recesses 64 in its opposite ends having their inner surfaces flush with the end surfaces 38 of the Through these recesses the terminal leads are This encapsulating material seals the regions 66 within the recess 64 adjacent to each terminal lead.

During the fabrication of each of the resistors 20, 20A,

20B, 20C, or 20D, after the resistance wire has been buttwelded at one end to the transition element, the winding is subjected to a suitable aging procedure for stabilizing its resistance characteristics before the other end of the winding is butt-Welded to the other transition element. A

suitable aging procedure for obtaining this stabilization includes the following steps:

(A) 1Maintain 2 hours at C.

2Maintain 2 hours at 50 C. 3-Maintain 2 hours at 100 C.

Repeat these three steps in sequence five times each.

Repeat these two steps in sequence five times each.

(C) Repeat A steps in sequence five times each. (D) Allow to stand at room temperature for 2 months.

From the foregoing it will be understood that the en- 'capsulated precision resistors embodying the present invention, as described above, are well suited to provide the advantages set forth, and since many possible embodiments may be made of the various features of this invention and as the method and apparatus herein described may be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense and that in certain instances, some of the features of the invention may be used With- 'out a corresponding use of other features, all without departing from the scope of the invention.

I claim:

1. An encapsulated wire-wound resistor comprising a cylindrical bobbin having an axial opening at each end, a pair of flexible wire terminals extending axially from opposite ends of the resistor and each having its inner end cemented into the bore of the bobbin, a pair of cross pieces of resistance wire each secured to one of said wire terminals and being cemented in position transversely of the end of the bobbin, thereby forming an anchor for each terminal wire, a resistance wire wound on the bobbin and having its ends butt-welded to the respective cross pieces at each end of the bobbin, a sleeve of insulating material surrounding the bobbin and projecting beyond the bobbin at each end, thereby forming a cup at each end of the bobbin, and a plug of encapsulating material filling each cup and surrounding the cross piece and portion of the terminal wire adjacent thereto.

2. An encapsulated wire-wound resistor comprising a cylindrical bobbin having an axial opening at each end, a pair of flexible Wire terminals extending axially from opposite ends of the resistor and each having its inner end cemented into the open end of the bore of the bobbin, a pair of cross pieces of resistance wire each secured to one of said wire terminals and being cemented in position transversely of the end of the bobbin adjacent to the open end of the bore, thereby forming an anchor for each terminal wire, said cross pieces having a diameter commensurate with the diameter of the Wire terminals, a resistance Wire wound on the bobbin and having its ends butt-welded to the respective cross pieces at each end of the bobbin, a sleeve of insulating material surrounding the bobbin and projecting beyond the bobbin at each end, thereby forming a cup at each end of the bobbin, and a plug of encapsulating material filling each cup and surrounding the cross piece and portion of the terminal twire adjacent thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,265,821 Siegel Dec. 9, 1941 2,407,171 McFarren Sept. 3, 1946 2,460,795 Warrick Feb. 1, 1949 2,547,405 Mitchell et al. Apr. 3, 1951 2,685,016 Blackburn July 27, 1954 2,745,930 Reisman May 15, 1956 2,817,738 Kohler Dec. 24, 1957 2,844,692 Berkelhamer July 22, 1958 

